THIS invention relates to a process for separating olefins and paraffins from oxygenates in a liquid hydrocarbon stream.
The reaction of synthesis gas in a Fischer-Tropsch reactor at elevated temperature and pressure over an Fe or Co catalyst produces a range of hydrocarbons including paraffins, olefins and alcohols, with carbon chain length varying from 1 to greater than 100. The Fischer-Tropsch reaction can occur at a temperature in excess of 300° C. Generally, the operating temperature is in the range of 200° C. to 260° C. utilising fixed bed or slurry phase reactors. The use of a Fe/Mn/Zn catalyst operated at a pressure of 30-60 bar has been shown to display high selectivity to olefins and oxygenates (mainly alcohols) with chain lengths of from 2 to greater than 30.
In a prior art processes for the extraction of alpha olefins from Fischer-Tropsch product streams, the concentration of the desired alpha olefin is increased in a first distillation step, followed by etherification, extractive distillation, adsorption and/or superfractionation steps. South African patent no. 98/4676 teaches an improvement in the process for the recovery of alpha-olefins from a mixed hydrocarbon stream derived from the Fischer-Tropsch process. The process described in this prior art involves three distinct steps:                1) Coarse separation of hydrocarbon products into a low boiling and a high boiling fraction        2) Distillative fine separation of lower and higher boiling fractions        3) Etherification of tertiary olefins following the fine separation. In these processes, tertiary olefins were considered as a problematic component that cannot be separated from the desired alpha olefin readily by conventional distillation in order to produce a product that is of polymer grade.        
A known commercial process of removing oxygenates from a hydrocarbon stream includes the hydrogenation of a C10 to C13 cut of a hydrocarbon stream containing olefins, paraffins and oxygenates. The hydrogenation step removes oxygenates, but also hydrogenates olefins to paraffins, which is undesirable.
U.S. Pat. No. 4,686,317 discloses a process for removing oxygenated impurities from a light (C2 to C4) hydrocarbon stream which includes extracting the oxygenates with a heavy organic polar solvent, water scrubbing the extracted hydrocarbons to recover the dissolved solvent, and combining the solvent phase from the extraction and water phase from the scrubber and distilling to recover the solvent. This process is not concerned with the recovery of oxygenates from the extract.